1. Field of Invention
This invention relates to holography, and more particularly to volume holography.
2. Description of the Prior Art
A hologram is a diffraction grating having bars corresponding to fringes of an interference pattern formed by two coherent beams of light. The beams of light are referred to as an object beam and a reference beam. The hologram is formed by photographically recording the interference pattern, whereby the hologram corresponds to the interference pattern.
Typically, rays of the object beam are either shifted in phase or amplitude modulated to provide a representation of an image. Light rays comprising a holographic reconstruction of the image emanate from the hologram in response to a transmission thereto of a reconstructing beam of light.
In a volume hologram, the diffraction grating extends within a substantial portion of a recording medium which is much thicker than the spacing between adjacent bars of the diffraction grating. Unlike other types of holograms, the holographic reconstruction of the image emanates from the medium only when the reconstructing beam is transmitted along the same path over which the reference beam was transmitted during the recording of the volume hologram. Therefore, the volume hologram is angularly sensitive to the reconstructing beam. Because of this angular sensitivity, the recording medium may be used for recording a multiplicity of volume holograms as explained hereinafter.
To record a book, for example, the medium is exposed to record a volume hologram of an image of the first page of the book. After the recording of the first page, the medium is incrementally rotated (through an angle of one degree, for example) with respect to the reference beam. Thereafter, the medium is exposed to record a volume hologram of an image of the second page of the book and the medium is incrementally rotated as described hereinbefore. In a similar manner, as many as 360 pages of the book are recorded on the medium.
Since the medium is incrementally rotated after an exposure, a reconstructing beam causes a holographic reconstruction of an image of only one page at a time to emanate (from the medium). Accordingly, the medium may be rotated with respect to the reconstructing beam to cause a holographic reconstruction of an image of a desired page to emanate.
Typically, the recording medium is either a rectangular or a cylindrical solid. When the images are recorded on the rectangular solid, reconstructions thereof are spatially displaced from each other. Additionally, corners of the rectangular solid cause distortion of some of the reconstructed images. When the images are recorded on the cylindrical solid, the curvature thereof causes a focusing of the object and reference beams on a focal surface within the cylindrical solid, thereby causing distortion of the stored and the reconstructed images. Therefore, most recording mediums known in the prior art are unsuitable for recording a multiplicity of volume holograms.